REVISION HISTORY: Transcribed from the microfilm by Kees Rookmaaker, corrections and editing by John van Wyhe 6.2010. RN1

NOTE: This document, part of the largest scientific document composed by Darwin during the voyage of the Beagle, is written mostly in ink. Marginal notes are here integrated into the text.

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Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin.

Having revisited P. Alta, seeing the neighbouring country. my opinion respecting its geology is completely altered (a) — & renders superfluous the greater part of the following pages. — The P. Alta bed is not coeval with the great Tosca formation: this is clear from the Tosca, containing shells, from the presence of Quartz pebbles, from & those of Pumice & (chiefly) rolled masses of Tosca rock. Its present position is owing to the last elevation, which this country has undergone. — for I cannot quite think, it has been formed under present circumstances; there is no doubt that such a bed of gravel might easily be thrown up; but these alternations of Tosca. with horizontal variations in colour & arrangement of pebbles do require some little depth of water; many other phenomena. require are best explained by small modern upheaval: — mud banks: — V following pages compare Tosca plain. with fragments of shell behind coast sand dunes (b) (this indubitably must have been formed beneath water & no change of tides &c &c would be sufficient to produce it): the higher sand-dunes, without shells, behind plain: & lastly the basin, in which the Fort stands, this is composed of swamps & marshes, & lines of low sand dunes, with small Turbo's (such as now exist on mud bank); all without any appearance of the, & the detritous plain of which the Town stands, it is composed of Tosca tock. in a whitish marl with fragments of quartz. — now the great Tosca formation never contains any pebbles; this is level & about 30

(b) The Pumice, which is in the gravel at P Alta & Mt: Hermoso. & scattered over the low impure Tosca plains; must have been deposited, when the country was beneath the sea, previous to last upheaval. — We have however plenty of instances of Pumice being now brought from the Andes. — On the banks of the R. Negro I saw much & the same officers in the small Schooner. found scoriæ &c in the R Chupat

feet above the swamps; therefore posterior to true Tosca & formed under water. — The basin is surrounded by cliffs, (a) presicely such as wood surround a bay with a sea. & such I believe it was when. P. Alta was formed. — With respect to the age of M: Hermoso, I am doubtful: the absence of shells & pebbles in Tosca, the presence of Tosca rock concretions, the grouping of the bones widely separates this bed from P. Alta. — I do not however think it contemporaneous with the great Tosca formation chiefly from the soft sandstone with quartz pebbles; — perhaps it may be coeval with the Tosca of B. Oriental & that at Bajada, which overlies the Limestone & which does not generally contain so much calcareous Tosca rock. & has so many bones. — From the scattered rolled state of the bones at P. Alta. & from one small pieces (1541) 1541; like those jet ones of M: Hermoso. I have no doubt many of 5 bones owe their origin to such beds as those of M: Hermoso. — All the bones however cannot do so, for instance, the most perfect skeleton (1405) 1405 (X) (L), must have been washed entire to near the place where found. & therefore a very modern animal coeval with present shells & posterior to great Tosca formation. —

The chief thing proved by this place, is an upheaval posterior to great Tosca plain; & to the coevality of certain animals at. M: Hermoso & the extreme newness lateness of existence of Animal (1405). —

Generally the P. Alta bones can prove nothing; nor indeed the M: Hermoso with reference to other formations. —

(a) Also the sand dunes above the edge of cliff on South side of Basin. —

(b) This skeleton was not in the gravel, like the other bones, but held together in the agglutinated sand it was 1/2 imbedded in loose sand at low water: the different parts of skeleton were going nearly in their proper part of position; I suppose this agglutinated sand is part of the gravel formation. — & the whole mass broke from some part of the cliff or bed & left where it now stands

This animal in all probability lived with those which now exist: for this last & small elevation must have been very recent & previous to this the animals body was washed into a shallow sea. —

It may strongly be suspected that the large piece of Megatherium hide, with some bones, could not be owing to the action of the sea on previous cliffs.

(c) I do not think it is quite certain, but that the lower beds, with quartz granules, may be original formation, differing from the superior beds, as at S. Cruz. —